1. Field of the Invention
The present Invention relates to an optical transmission system which transmits a signal via a plurality of optical repeaters each including an in-line optical amplifier such as an optical fiber amplifier and, more specifically, to a system for transferring repeater information signals which are respectively generated in the optical repeaters.
2. Description of the Prior Art
In a multiple-repeater optical transmission system that transmits an optical signal via a plurality of repeaters, some failure may occur in a transmission line or a repeater, resulting in a possible serious failure in the transmission system. Therefore, it is necessary to always supervise the repeaters and transmission lines and to immediately inform a receiving terminal and a control terminal of a failure upon its detection.
An example of a supervisory method in an optical transmission system Is disclosed in Japanese Unexamined Patent Publication No. 2-266245. In this method, an Er(erblum)-doped fiber is used as an in-line optical amplifier, and pumping light is modulated by a pulse pattern whose period is shorter than the relaxation time of pumping carriers. This pumping light modulation causes the pulse pattern to be sent to an optical fiber transmission line. An optical loss and a failure position of the optical fiber transmission line is determined by calculating a time correlation between the above pulse pattern and a pulse pattern of backscattered light returning from the optical fiber transmission line. Although this type of supervisory method is effecting in supervising the optical fiber transmission line, it cannot provide effective information when a failure occurs in a repeater.
By analogical inference from this supervisory method using the pumping light modulation, a system may be conceivable in which each repeater is provided with a supervisory device and failure information in each repeater is superimposed on a main optical signal and is transmitted downstream. More specifically, a carrier of a predetermined frequency is digitally modulated in accordance with the failure information, and pumping light for the Er-doped fiber is generated in accordance with the modulated carrier. Accordingly, this system can achieve both optically amplification of the main signal through the Er-doped fiber pumped by the pumping light and transfer of the failure information carried by the pumping light in a state of being superimposed on the main signal.
However, the above failure information transfer system has the following problem. Since the modulation frequency for superimposition of failure information is the same for all the repeaters, when failures occur simultaneously in two or more repeaters, respective pieces of failure information cannot be transferred being superimposed on the main signal. Therefore, it is difficult to identify two or more failure repeaters, respectively. This is a factor of reducing the reliability of the optical transmission system.